1. Field of the Invention
This invention relates generally to packaging of electronic circuit device components. More particularly, the invention pertains to packaging arrangements for such device components mounted on an array substrate, though not limited to a method for encasing or covering such electronic devices. The invention further pertains to packaging arrangements for such device components mounted on an array substrate and devices for enclosing or sealing such components.
2. State of the Art
Modern packaged integrated circuits (IC) comprise one or more encased semiconductor devices or chips within a protective "package" of plastic, ceramic, moldable material, or metal or other preformed material, such as caps. The integrated circuit chips are made from a semiconductor material such as silicon, germanium or gallium arsenide, and microscopic circuits are formed on a surface of each chip surface by photolithographic techniques. A plurality of external connections, typically designed for soldering or slide connections, are connected to bond pads on one or more encased chips, enabling the chips to be electrically interconnected to an external electrical apparatus. In one form of interconnection, a substrate such as a wiring board or circuit board has an array of conductors which is typically connected to the wire bond pads of the chips. Portions of the conductors extend through the substrate, typically in through-holes or vias to the opposite side for conductive, e.g. solder, connection to another electronic apparatus. In addition to one or more semiconductor devices (chips or dies) attached to the substrate, or in lieu thereof, other devices such as resisters, capacitors, etc., as well as the conductive leads and wires, may be mounted to the substrate and incorporated in the circuit. Such elements are encased in plastic, ceramic or other material for protection.
Plastic encapsulation of semiconductor and other electronic devices by transfer molding is a well-known and much-used technique. Typically, a large number of components or devices is placed in a lower mold plate or half of an open multi-cavity mold, one device within each cavity. The mold is closed with a mating upper plate. The cavities of the mold are connected by tiny "feed runners", i.e. channels to a "pot" or reservoir from which pressurized liquified plastic is fed. Typically, a constricted channel known as a "gate" is located at the entrance to each mold cavity to limit the flow rate and injection velocity of liquified plastic into the cavity.
Where it is desired to encase the electronic components mounted on one side of a circuit board or wiring board, while leaving uncovered an array of terminals on the opposite side, a peripheral portion of the board (or of a portion encompassing a mounted circuit) is clamped and compressed between the upper and lower mold plates to prevent leakage of liquified plastic from the one side of the mold cavity. Typically, the force required to compress the plates together is of the order of tons, even for molding machines having only a few mold cavities.
Typically, powdered or pelletized plastic, e.g. thermoset resin, is placed in the resin pot and pressed by a ram. The heated, pressurized plastic becomes liquified and flows through the feed runners and gates to surround each device on one side of the substrate and fill that portion of each mold cavity, where it subsequently hardens to encapsulate one side of the board and the devices attached to it. Air is expelled from each cavity through one or more vent runners as the plastic melt fills the mold cavities. Following hardening by partial cure of the thermoset plastic, the mold plates are separated along the parting line and each encapsulated device is removed from a mold cavity and trimmed of excess plastic which has solidified in the runners and gates. Additional thermal treatment completes curing of the plastic package.
Following removal of each encased unit from its mold cavity and curing, the peripheral portions of the board may be excised from the board and any flash, i.e. plastic or other extraneous material removed from external terminals, etc. as known in the art, and the device is ready for use.
In devices having one side of the substrate configured for a ball grid array (BGA) or similar array on a circuit board, the molding process is conducted so that the surface of the circuit board having the ball grid array connections are formed on an outer surface of the package, such surface not being covered or encapsulated by the plastic material during the encapsulation process. When the substrate is sealably clamped on all sides of the cavity, plastic may reach the ball grid array side of the substrate only through the substrate, e.g. inadvertently through a hole or via. Of course, following removal from the cavity, any plastic encapsulant which may have reached and solidified on the ball grid array connection surface is removed.
The encapsulation process is typically performed before the "balls" of solder are placed on the pads of the grid array, in order to prevent possible inadvertent disforming or loss of any solder balls during encapsulation.
As disclosed in the prior art, various integrated circuit devices are configured for one-side enclosure or encapsulation, with an opposing bare or exposed side. U.S. Pat. No. 5,598,034 to Wakefield discloses an electronic device having a lower bare surface of a metallic heat conductor to prevent overheating of the integrated circuit.
U.S. Pat. No. 5,608,262 of Degani et al. shows different semiconductor devices in which a printed wiring board surface or semiconductor chip surfaces are left uncovered.
In U.S. Pat. No. 5,222,014 of Lin, a stackable multi-chip module (MCM) is shown having several levels of chip-carrying substrates with accompanying ball-grid-arrays of terminals.
U.S. Pat. No. 5,615,089 of Yoneda et al. teaches the use of a first substrate carrying chips on both surfaces, and a second substrate carrying the first substrate, wherein the second substrate has a bare surface with arrayed terminals.
In U.S. Pat. No. 5,609,889 of Weber, a mold is described which has a biased plug that exerts pressure on a heat sink or circuit board to prevent molding compound from covering its surface. A passage is provided in the substrate circuit board so that plastic flows latitudinally under the circuit board into a cavity. The plug is biased by a plate spring to accommodate variations in the thickness of the substrate and ensure that the exterior surface of the heat sink does not become significantly encased in plastic.
In each of these references, the device is one-side encapsulated in a set of mold plates, one to a mold cavity.
U.S. Pat. No. 5,313,365 of Pennisi et al. discloses an electronic conductor-grid-array package including integrated circuits bonded to one side of a printed circuit board, and a grid array on the opposing side. Instead of using transfer molding techniques, the integrated circuits and associated wiring are encased in a glob-top encapsulant Typically, glob-top encapsulation is more time consuming, less reliable, and yields a product having a less pleasing appearance than conventional transfer molding methods.